Just wondering, if when playing a game of tunnels with Smallworld and Smallworld Underground, what happens if the necromancer gets control of a relic or a place, do they score, like with the Mine of the lost dwarf, will he get two extra bonus coins or just deprive others of the ability?

I think that relics and places work as usual for Nercomancer. Maybe there are some that must be removed from the game if Necromancer can't profit from them (if there are any like this - I haven't analyze them).

It is stated in the rules that the Necromancer must be considered as an in-decline race. So he can't use Relics nor Places (unless the rules specify that the said Relic or Place still works for in-decline races).

Actually the wording in the rule pamphlet says:Other players always consider Ghosts (not the Necromancer player) as In Decline...
But then it goes on to further confuse the issue with the following example:...So they cannot strike a Diplomatic pact with them. (This after DOW has gone to lengths to clarify that the Diplomat player targets a player, not the player's race.)
This isn't the only instance where the Necromancer Island rules are lacking or contradictory.

I don't have anything to add in terms of answering the OP's question about Relics/Places when mixed with the Necromancer. With the re-release of NI due to the recent Kickstarter campaign, I'm sure we'll see a flurry of these questions roll in.